If you would like to buy Differin gel in a country where it is not over the counter such as the UK, you could buy it on iHerb. It is a US site which ships to other countries, I got some Differin gel from there shipped to the UK and it was less painful than figuring out how to get American Amazon or Walmart to ship abroad. But it is more expensive than the Amazon link OP provided, so I guess this is more of a thing if you want to try some out and see if it works for you with as few effort as possible (although beware that it doesn't work immediately, you will probably have to wait a couple months to start seeing results).
If you would like to buy Differin gel in a country where it is not over the counter such as the UK, you could buy it on iHerb. It is a US site which ships to other countries, I got some Differin gel from there shipped to the UK and it was less painful than figuring out how to get American Amazon or Walmart to ship abroad. But it is more expensive than the Amazon link OP provided, so I guess this is more of a thing if you want to try some out and see if it works for you with as few effort as possible (although beware that it doesn't work immediately, you will probably have to wait a couple months to start seeing results).